This invention relates to an electrochemical element in general and to an oxygen sensor element in particular, which is used to determine the concentration of oxygen contained in exhaust gases from an internal combustion engine, for instance.
So far, there has been known an electrochemical sensor element including first and second electrode layers and a solid electrolyte, wherein the first and second electrode layers are brought into contact with the gas to be measured and a reference gas, respectively, whereby a potential difference between the first and second electrode layers is found to determine the concentration of a specific component, e.g., oxygen, in the gas to be measured.
One typical oxygen sensor is set forth in Japanese Patent Kokai Publication No. 60(1985)-36948, and includes a first electrode layer which is covered on its portion to contact the gas to be measured with a single porous protecting layer. Another oxygen sensor is set forth in Japanese Patent Kokai Publication No. 60(1985)-36949, and includes a first electrode layer which is covered on high- and low-temperatures regions of its portion to contact with the gas to be measured with a porous protecting layer and an air-tight protecting layer, respectively.
However, a problem with the oxygen sensor having the first electrode layer covered with a single protecting layer is that when the porous protecting layer has an increased gas permeability, carbon is likely to accumulate on and in it and in the first electrode layer while the internal combustion engine operates under conditions rich in fuel or, in other words, exhaust gas conditions poor in oxygen, giving rise to a lowering of insulation resistance between the first electrode layer and the metal holder of the oxygen sensor. In particular, this decrease in insulation resistance become more serious when carbon deposition takes place on the low-temperature region of the porous protecting layer or the first electrode layer in an arrangement where the first electrode layer lies relatively close to the metal holder.
A problem with the process of producing the oxygen sensor set forth in Japanese Patent Kokai Publication No. 60-36949 is that when the oxygen sensor element is immersed in a solution, the solution penetrates through the porous electrode layer so that it is evaporated (gasified) rapidly during baking; that is, if a portion of the protecting layer covering the first electrode layer is air tight, then the gas to be evaporated is so confined within the pores in the first electrode layer that the internal pressure of the first electrode layer increases, causing the protecting layer to peel off or crack.
This invention seeks to provide a solution to the above problems and has for its object to provide an electrochemical element which can be produced without detriment to the porous protecting layer or, in other words, without causing it to peel off and crack, and which can be used while any lowering of insulation resistance between the first electrode layer and the metal holder is avoided, thereby providing an accurate detection of the concentration of a specific component in the gas to be measured.